Wavelength division multiplexing (WDM) is applied to achieve high-capacity optical communications. The WDM technology employs a tunable light source (TLS) that oscillates light of different wavelengths. In some cases, a wavelength monitor for wavelength control may be provided in the tunable light source. The wavelength monitor is implemented by, for example, a wavelength filter having a constant periodic transmission spectrum and a photodiode (PD). Light having different wavelengths may be obtained by the periodic transmission spectrum. A delay interferometer is used as a wavelength filter of the periodic transmission spectrum.
In order to control the wavelength with high precision, it is desirable for the wavelength filter of the wavelength multiplexing light source to have a free spectral range (FSR) of approximately one nm or less. The FSR is determined based on the delay amount of the delay interferometer.
In the related art WDM technologies, tunable light sources based on a planer lightwave circuit (PLC) having an optical circuit formed on a quartz substrate have been used. In view of down-scaling devices, it is desirable to form a resonator or a wavelength filter by silicon photonics technology. In a case where the delay interferometer is formed of a silicon (Si) waveguide, an FSR of 1 nm may be obtained by setting the difference between the two arm lengths to approximately 0.5 to 1 mm.
However, the thermo-optic coefficient of silicon (Si) is larger than that of the PLC waveguide, and the delay amount is likely to change due to temperature change. The thermo-optic coefficient represents temperature dependence of optical properties such as refractive index. When Si, which has a larger thermo-optic coefficient than quartz is used, the peak wavelength of the wavelength filter tends to shift due to temperature change.
FIG. 1 illustrates a related art configuration example for canceling a change in delay amount due to a temperature change. Of the two waveguides 112 and 113 forming the delay interferometer 110, the core width Ws of the shorter waveguide 112 is made thicker than the core width Wl of the long waveguide 113. This configuration allows the difference in temperature dependency change occurring between the two waveguides 112 and 113 to be canceled out to make the temperature dependence of the optical lengths approximately the same (see, e.g., Patent Document 1 and Patent Document 2).